The demand for increased bandwidth in fiberoptic telecommunications has driven the development of sophisticated transmitter lasers usable for dense wavelength division multiplexing (DWDM) systems wherein multiple separate data streams propagate concurrently in a single optical fiber. Each data stream is created by the modulated output of a semiconductor laser at a specific channel frequency or wavelength, and the multiple modulated outputs are combined onto the single fiber. The International Telecommunications Union (ITU) presently requires channel separations of approximately 0.4 nanometers, or about 50 GHz, which allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Greater bandwidth requirements will likely result in smaller channel separation in the future.
DWDM systems have largely been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels.
Continuously tunable external cavity lasers have been developed to overcome this problem. The advent of continuously tunable telecommunication lasers has introduced additional complexity to telecommunication transmission systems, as individual lasers must be able to provide stable, accurate tuning at narrowly separated channel wavelengths. Particularly, the tuning aspects of external cavity lasers involve multiple optical surfaces that are sensitive to contamination and degradation during use. Further, external cavity laser tuning typically involves drive elements that can introduce contaminants to the laser optical surfaces. These deficiencies have resulted in increased costs and decreased performance lifetimes for tunable telecommunication transmitter lasers.